If lithium drugs decrease memory, why it’s used in Alzheimer’s’s patients?

It has been told that lithium may have neuroprotective abilities, but it is not known whether lithium decreases the risk of dementia. People who have purchased lithium at least once had an increased rate of dementia compared with people not exposed to it.
For individuals who continued to take lithium, the rate of dementia declined to the same level as the rate for the general population. The rate began to decline after taking lithium tablets corresponding to 1 prescription (commonly 100 tablets) and remained low regardless of the number of subsequent prescriptions. The link between the number of prescriptions for lithium and dementia was unusual and different from the connection between the number of prescriptions for anticonvulsants and dementia.
Continued lithium treatment was linked with reducing the rate of dementia to the same level as that in the general population. Methodological explanations for this finding cannot be excluded, owing to the non-randomized nature of the data.
Studies indicate that bipolar disorder is associated with an increased risk of developing dementia and that the risk rises with every episode of the disorder.
On the other hand, it has been proposed that lithium may have neuroprotective abilities and may decrease the risk of developing dementia because lithium interferes with glycogen synthase kinase, which is a critical enzyme in the metabolism of amyloid precursor protein. It also inhibits the phosphorylation of the tau protein entangled in the pathogenesis of Alzheimer disease. Nevertheless, the link between lithium consumption and dementia has been studied in only two human studies.
One study indicated an increased risk of dementia with raising numbers of lithium prescriptions, whereas the other study discovered that patients receiving long-term lithium therapy had reduced prevalence of Alzheimer’s compared with patients not receiving lithium therapy. Results from both studies were hindered by the inclusion of low numbers of patients participating, including 87 and 114 patients, respectively.